Typically, a current transformer for generating power by using a magnetic field generated in a distribution line is required to be separately designed and manufactured by considering characteristics of the current transformer, such as the size and capacity thereof, according to an installment position, since line current on the distribution line is not constant in real time, and furthermore a change width is large according to the position thereof.
Moreover, as a desired power amount increases, the size of the current transformer tends to be increased, and in particular, the case for a separable current transformer, an increase in output power amount is costly and effortful by increasing the size of the current transformer.
Even when this separable current transformer is capable of being designed and manufacturing, since manufacturing an enclosure of the current transformer has many technical limitations and is costly, implementation of a power supplying device by using a separable current transformer is costly and limited.
In particular, since most of current transformers are mainly used as a sensor rather than a power generator, an increase in power output amount has been researched only in the point of view of improving a signal-to-noise ratio, and a research on a separable magnetic core as a power supplying device is still insignificant.
Furthermore, since the distribution lines have various minimum line currents according to an installment environment, power that the power supplying device may generate by using the current transformer is limited and accordingly the current transformer is required to be differently designed according to each environment. In addition, since the size of the current transformer varies according to a system using a minimally required power amount, lots of time and expense are required to obtain desirable power amount.
Hereinafter, difficulty in design of an output of magnetic induction power supplying device will be described in relation to FIGS. 1 to 2D. FIG. 1 is a conceptual diagram of a power supplying device for the distribution current, and FIGS. 2A to 2D are perspective views of a separable magnetic core.
As illustrated in FIG. 1, the power supplying device for distribution line includes a current transformer CT inducing AC current by primary current I flowing through the distribution line, and a rectifier converting the induction voltage corresponding to the AC current induced by the current transformer CT into a DC voltage. At this point, an output voltage Vo of the current transformer may be determined according to the size of the magnetic core.
Furthermore, the current transformer may use a separable magnetic core in consideration of ease of installation and removal, or as shown in FIG. 1B, may use identical or similar magnetic cores in plural. For example, FIG. 1C shows a separable magnetic core having the inner diameter of 44 mm, the outer diameter of 75 mm, and the length of 90 mm. FIG. 1D shows two separable magnetic cores having the inner diameter of 44 mm, the outer diameter of 75 mm, and the length of 45 mm, and the two separable magnetic cores are combined to have identical dimensions to those shown in FIG. 1C.
In addition, as the required output power increases in order to increase power induced by a current transformer, it is better to use a magnetic core having dimensions as large as possible, for example, having a longer length, as shown in FIG. 1A or 1C. However, this has limitations as follows.
First, for general characteristics of a magnetic core, the magnetic inductance L generated in the magnetic core by line current, and a resonant frequency f of a current transformer at this point are expressed as the following Equations (1) and (2).
                    Equation        ⁢                                  ⁢        1                                                            L        =                                            4              ⁢                                                          ⁢              π              ⁢                                                          ⁢                              μ                r                            ⁢                              n                2                            ⁢              S                        l                    ⁢                                    10                              -                7                                      ⁡                          [              H              ]                                                          (        1        )                                Equation        ⁢                                  ⁢        2                                                            f        =                  1                      2            ⁢                                                  ⁢            π            ⁢                          LC                                                          (        2        )            
where, μr denotes the relative permeability of the magnetic core, l denotes the length of a magnetic field loop in the magnetic core, n denotes the number of windings of coil wound around the magnetic core, and S denotes a cross-section area of the magnetic core.
As may be seen from Equations (1) and (2), in order to increase a magnetic field induced in the magnetic core, it is required to increase the number of windings of coil and dimensions of the magnetic core, for example, the cross-section area and relative permeability of the magnetic core. However, this results in increases in magnetic inductance and capacitance and decreases the resonant frequency f. In particular, the resonant frequency f becomes close to frequency of line current, namely 60 Hz (or 50 Hz) at the time of power-on, which results in losing a function as the power supplying device.
In addition, from the point of view of manufacturing the magnetic core, as the dimensions become larger, the manufacturing cost increases and accordingly an enclosure design cost also greatly increases. Accordingly, it is cheaper to manufacture the magnetic core of a size shown in FIG. 2D, the resonant frequency of which is considered, and to obtain the same effect as that shown in FIG. 2C. In other words, it is better to manufacture the current transformer having a unit size shown in FIG. 2D to increase power. However, even in this case, since output power does not increase in proportion to addition of the current transformer having a unit size, it is difficult to design a system that matches a magnitude of the output.
In detail, the magnitude φ of magnetic flux, which excites the coil winding the magnetic core, is expressed as Equation (3).
                    Equation        ⁢                                  ⁢        3                                                            ϕ        =                                            μ              0                                      2              ⁢                                                          ⁢              π                                ⁢                      μ            r                    ⁢          WI          ⁢                                          ⁢                      ln            ⁡                          (                              1                +                                  h                  r                                            )                                                          (        3        )            
where W denotes the width of the magnetic core, h denotes the height of the magnetic core, μr denotes the a relative permeability of the magnetic core, and μ0 denotes a vacuum permeability expressed as μ0=4π10−7(H/m).
At this point, a voltage induced at a terminal of the coil is expressed as Equation (4).
                    Equation        ⁢                                  ⁢        4                                                            v        =                              -            N                    ⁢                                    d              ⁢                                                          ⁢              ϕ                                      d              ⁢                                                          ⁢              t                                                          (        4        )            
where N denotes the number of windings of the coil.
A root mean square (RMS) value of the induced voltage is expressed as Equation (5) by using Equations (3) and (4).
                    Equation        ⁢                                  ⁢        5                                                                      v          r                =                  NIWf          ⁢                                          ⁢                      μ            0                    ⁢                      μ            r                    ⁢                      ln            ⁡                          (                              1                +                                  h                  r                                            )                                                          (        5        )            
where f is a frequency of induction current.
Although not including a modeling of an air gap generated in the cross-section of the separable magnetic core, the above Equations are sufficient to analyze an overall operation of the current transformer.
At this point, the line current I of the distribution line is expressed as Equation (6) and the induction current of the magnetic core by the line current is expressed as Equation (7).Equation 6I=I0 cos(2πft)  (6)Equation 7i=i0 cos(2πft+θ)  (7)
where θ denotes a phase difference between the excitation voltage and excitation current, which is because the coil functions as an inductive load and a capacitive load on an AC line and accordingly the excitation voltage and excitation current have different phases.
Finally, the induced power in the coil is expressed as the Equation (8).Equation 8P=ν0i0μr cos(θ)  (8)
Here, according to usage degrees by users, maximum values ν0 and i0 of induction voltage and current changes in real time, and accordingly μr changes. In addition, the magnetic φ of magnetic flux induced in the magnetic core also changes and resultantly, the amplitudes of the induction voltage and current, and phase difference θ also change. Accordingly, although an output of the magnetic core is added, the output power does not become doubly increased by differences in the size and phases of the induced voltage and current.
FIG. 3 is a graph representing output power according to the number of typical current transformers connected in serial.
As represented in FIG. 3, when the magnetic cores are simply connected in serial, an output power amount increases according to an increase in current of the distribution line but does not increase in proportional to the number of magnetic cores connected in serial.
In order to address this issue, except for changes in maximum values of the induction voltage and current generated by at least a magnitude change rate of line current, reduction in the maximum output generated by the phase difference of the voltage and current should be prevented.
Furthermore, the power supplying device in a separable magnetic core type, which uses induction power using a magnetic field, is a power supply device as itself, and generally enables an increase in power by increasing the size of the magnetic core or connecting a plurality of small cores in serial. However, as disclosed in Korean Patent Application Laid-open Publication No. 10-2009-0088179, an increase in the number of magnetic cores does not allow the induction voltage to be increased proportionally to the output power.
Accordingly, the following requirements are to be satisfied in order to realize the power supplying device by using the separable magnetic core.
(1) An output power amount is required to be easily handled according to a current magnitude of the distribution line.
(2) Desirable output power is required to be easily achieved only by just adding a current transformer regardless of a minimum current magnitude of the distribution line.
(3) An enclosure design is required to be easy regardless of a desirable output amount and the separable current transformer is required to be easily manufactured.
(4) The size of the separable current transformer is required to be determined as a size of not being influenced by a resonant frequency.